The present invention relates to a push button switch which is attached to a control panel of a machine tool or the like, and used to give an emergency stop instruction etc. to the machine tool or similar instrument.
A push button switch for emergency stop is generally formed by combining and coupling a switch portion and an operation portion. The switch portion has a switching contact unit constituted by a fixed contact and a movable contact which is opened/closed against the fixed contact. The operation portion has a push button operation mechanism for performing an operation of opening/closing the switching contact unit.
FIGS. 28A-36 illustrate an example of a convention push button switch configuration. FIGS. 28A and 28B are perspective views showing the external appearance of the push button switch. FIGS. 29A-32B are sectional views of the push button switch shown in FIGS. 28A and 28B, taken on different cutting lines respectively.
In the drawings, the reference numeral 1 represents an operation portion in which an operation mechanism is received in an operation portion case 11. The operation mechanism includes a push button 12, a push rod 13, etc. The push button 12 can be operated to be pushed in. The push rod 13 is linked with the push button 12 so as to work together therewith.
The reference numeral 2 represents a switch portion which receives a switching contact unit 24a and a switching contact unit 24b. The switching contact unit 24a has a normally open contact (“a” contact) configuration including a fixed contact 22a and a movable contact 23a which are normally open. The switching contact unit 24b has a normally closed contact (“b” contact) configuration including a fixed contact 22b and a movable contact 23b which are normally closed. Each switching contact unit has a movable frame 25a, 25b which slides supporting the movable contact 22a, 22b. 
The operation portion 1 and the switch portion 2 are formed separately as shown in FIG. 29. A fitting hole 21a is provided in a case 21 of the switch portion. A body portion 11a at a leading end of a cylindrical case 11 of the operation portion 1 is fitted into the fitting hole 21a. A hook 21c provided in the inner circumference of the fitting hole 21a of the switch portion case 21 is engaged with an engagement recess 11c provided in the outer circumference of the body portion 11a so that the operation portion 1 and the switch portion 2 are coupled and combined integrally. To release the operation portion 1 and the switch portion 2 from the coupling, a slider 29 provided in the switch portion 2 is operated to release the hook 21c from the engagement with the engagement recess 11c. 
FIGS. 29A to 32B show sectional views taken on cutting lines XIX and XXII in FIG. 28B respectively. Each FIG. 29A-32A shows a standby state in which the push button 12 of the push button switch has not been pushed in but is put in its initial reset position. Each FIG. 29B-32B shows an operating state in which the push button 12 has been pushed into an operating position in the lower side of the drawing.
The operation portion 1 is provided with a lock mechanism to keep the push button 12 pushed in. The lock mechanism is constituted by a protrusion 13c and a lock plate 11c. The protrusion 13c is provided in the outer circumference of the push rod 13. The lock plate 11c is provided in the inner circumference of the body portion 11a of the case 11. When the push button 12 is pushed in at the standby position where the push button 12 has not been pushed in, the push rod 13 linked with the push button 12 moves down in connection therewith. When the protrusion 13c of the push rod 13 gets over the lock plate 11c and reaches a push-in position, the protrusion 13c engages with the lower end of the lock plate 11c so as to prevent the push button 12 from returning. Thus, the push button 12 is kept in the push-in position even after it is released from operation. To return the push button 12 to the standby position from the push-in position, the push button 12 is rotated at a predetermined angle to release the lock plate 11c and the protrusion 13c of the push rod 13 from engagement so as to release the push button 12 from the lock. Due to a return spring 14 consisting of a torsion coil spring, the rotated push button 12 is returned to the reset position where the push button 12 had been put before the rotation.
In the state where the operation portion 1 and the switch portion 2 have been coupled, as shown in FIG. 29A, the movable frames 25a and 25b respectively supporting the movable contacts of the switching contact units of the switch portion 2 are pushed up by return springs 26a and 26b respectively. Thus, the upper end of each movable frame 25a, 25b is brought into contact with the lower end of the push rod 13 linked with the push button 12 of the operation portion 1. When the push button 12 is in the standby position, the movable contact 23a leaves the fixed contact 22a as shown in FIG. 31A. Thus, the switching contact unit 24a having the normally open contact (“a” contact) configuration is turned off. On the other hand, the movable contact 23b contacts with the fixed contact 22b as shown in FIG. 32A. Thus, the switching contact unit 24b having the normally closed contact (“b” contact) configuration is turned on.
Here, when the push button 12 is operated to be pushed down against the return spring 14, the movable frames 25a and 25b of the switch portion 2 are pushed down by the push rod 13 working with the push button 12, so that the movable contacts 23a and 23b move down together with the movable frames 25a and 25b. As a result, the movable contact 23a contacts with the fixed contact 22a as shown in FIG. 31B. Thus, the switching contact unit 24a having the normally open contact configuration is turned on. On the other hand, the movable contact 23b leaves the fixed contact 22b as shown in FIG. 32B. Thus, the switching contact unit 24b having the normally closed contact configuration is turned off.
The push button switch configured thus is in use attached to a panel such as a control panel. The process of attaching the push button switch will be described below. First, as shown in FIG. 33, a fixing nut 19 is detached from the operation portion 1 from which the switch portion 2 has been removed, and the body portion 11a is inserted into a mounting hole of the panel on the front side of the panel. Next, on the back side of the panel, the fixing nut 19 is screwed down to the inserted body portion 11a of the operation portion 1 so that the body portion 11a is fastened and fixed to the panel. The fitting hole 21a of the switch portion 2 is fitted to the body portion 11a of the operation portion 1 attached to the panel. Thus, the switch portion 2 is coupled and connected to the operation portion 1.
In the push button switch attached to a control panel or the like in use in this manner, there may occur an accident in which the switch portion 2 drops off the operation portion 1 for some reason when the push button switch is in use. When there occurs such an accident in which the switch portion 2 drops off the operation portion 1 in the state where the push button switch is operated to be pushed in, the mode in which the switch portion 2 is pushed in by the push rod 13 of the operation portion 1 is canceled so that the movable frames 25a and 25b of the switch portion 2 are returned to their reset positions by the return springs 26a and 26b. As shown in FIGS. 36 and 37, the normally open contact unit 24a is turned off again, while the normally closed contact unit 24b is turned on again.
Assume that the normally closed contact unit 24b is used for issuing an emergency stop instruction. When the normally closed contact unit is returned to be turned on due to an accident in which the switch portion 2 drops off the operation portion 1 in this case, the emergency stop instruction is cancelled so that there is a risk that a significant accident may be caused by resumed operation of an instrument which has been emergency-stopped.
In order to avoid such a risk, the opened movable contact of the switching contact unit having the normally closed contact configuration, which unit is used particularly as a switching contact unit for issuing an emergency stop instruction, has to be prevented from returning to the position where the movable contact will contact with the fixed contact, when the switch portion 2 of the push button switch drops off the operation portion.
As push button switches for solving such a problem, push button switches have been known in the background art as disclosed in Japanese Unexamined Patent Publication No. (Kokai) JP-A-2001-035302, Japanese Unexamined Patent Publication No. (Kokai) JP-A-2003-303527, Japanese Unexamined Patent Publication No. (Kokai) JP-A-2007-115703.
FIGS. 38-40 show the configuration of the conventional push button switch disclosed in JP-A-2001-035302. The conventional push button switch has an operation portion 1 and a switch portion 2. The operation portion 1 includes a push button 12, an operation portion case 11 and a push rod 13. The operation portion case 11 supports the push button 12 slidably thereon. The push rod 13 is provided in the operation portion case 11 so as to work with the motion of the push button 12. The switch portion 2 is removably attached to the lower portion of the operation portion case 11 of the operation portion 1.
The switch portion 2 has a fixed contact maker 22c and a fixed contact 22b. The fixed contact maker 22c has a folded portion with vertical flexibility. The fixed contact maker 22c is displaceably fixed to a switch portion case 21. The fixed contact 22b is disposed on the tip of the folded portion of the fixed contact maker 22c. The switch portion 2 also has a movable contact maker 23c and a movable contact 23b. The movable contact maker 23c is supported by a movable frame 25b. The movable frame 25b is urged toward the fixed contact 22b by a first spring 27 and urged away from the fixed contact by a second spring 28. The movable contact 23b is disposed on the tip of the movable contact maker 23c. 
When the operation portion 1 and the switch portion 2 configured thus are coupled with each other, the fixed contact 22b and the movable contact 23b of the switch portion 2 contact with each other to form an initial ON state (see FIG. 38).
When the push button 12 is pushed in during the initial ON state, a pressing force from the push rod 13 held in the push-in position by a lock mechanism 18 is applied to the folded portion of the fixed contact maker 22c through a second movable frame 25c. Thus, the fixed contact 22b is put away from the movable contact 23b to be brought into an OFF state (see FIG. 39).
When the switch portion 2 is released from the coupling and separated from the operation portion 1, the movable frame 25b supporting the movable contact 23b and the second movable frame 25c are pushed up by the effect of the elastic repulsion of the second spring 28. Thus, the movable contact 23b can be put away from the fixed contact 22b to form an OFF state (see FIG. 40).
The conventional push button switch disclosed in JP 2003-303527 is configured as shown in FIGS. 41 and 42, wherein an operation portion 1 has a push button 12, a push rod 13, a return spring 14, a lock mechanism 18, etc. The push button 12 is slidably supported by an operation portion case 11. The push rod 13 is linked with the push button 12. The return spring 14 serves to return the push button 12. The lock mechanism 18 serves to hold the push button 12 in a push-in position. A switch portion 2 receives a switching contact mechanism in a switch portion case 21 removably coupled with the lower end portion of the operation portion case 11 of the operation portion 1. The switching contact mechanism has a normally closed contact configuration constituted by a fixed contact 22b and a movable contact 23b. A movable contact maker 23c to which the movable contact 23b is attached is supported by a movable frame 25b. A spring 28 for urging the movable contact 23b away from the fixed contact 22b is provided between the movable frame 25b and the switch portion case 21.
When the switch portion 2 is coupled with the operation portion 1, the movable frame 25b is linked with the push rod 13 working with the push button 12 of the operation portion 1. The spring force of the spring 28 urging the movable contact 23b of the switch portion 2 away from the fixed contact 22b is made smaller than the spring force of the return spring 14 pushing up the push button of the operation portion 1. Thus, the spring forces of the respective springs are balanced to prevent the movable contact 23b from leaving the fixed contact 22b before the push button 12 is operated to be pushed in.
In the state where the push button is in the reset (initial) position where the push button has not been pushed in yet, the movable frame 25b of the switch portion 2 is lifted up by the push rod 13 of the operation portion 1 against the spring force of the opening spring 28 so that the movable contact 23b contacts with the fixed contact 22b to form an ON state, as shown in FIG. 41. As soon as the push button 12 is operated and pushed in as shown in FIG. 42, the push rod 13 is held in the push-in position by the lock mechanism 18. The movable frame 25b is pushed down as the push rod 13 is pushed in. Thus, the movable contact 23b leaves the fixed contact 22b to form an OFF state.
Once the operation portion 1 and the switch portion 2 are released from coupling with each other so that the switch portion 2 is separated from the operation portion 1, the movable frame 25b becomes free. Thus, the movable frame 25b is pushed down to put the movable contact 23b away from the fixed contact 22b by the spring force of the opening spring 28 so as to bring the switch portion 2 into an OFF state.
According to conventional push button switches disclosed in JP-A-2001-035302 or JP-A-2003-303527, an operation portion and a switch portion are removably coupled with each other. Once an accident occurs in which the switch portion drops off the operation portion in the push button switch, one of two closed contacts of a switching contact unit having a normally closed contact configuration is driven to be open to the other contact by an opening spring. Thus, the two contacts can be opened surely to form an OFF state.
However, according to the mechanism which opens the switching contacts of the switching contact unit having the normally closed contact configuration to form an OFF state as soon as the switch portion drops off the operation portion in the conventional push button switch, it is necessary to make both the contacts movable or to balance the spring force of the opening spring with the spring force of the return spring of the operation portion. Thus, there is a problem that the push button switch is too complicated in configuration to manufacture.